Efforts to improve upon the performance of natural mineral oil based lubricants by the synthesis of oligomeric hydrocarbon fluids have been the subject of important research and development in the petroleum industry for at least fifty years and have led to the relatively recent market introduction of a number of superior polyalpha-olefin (PAO) synthetic lubricants, primarily based on the oligomerization of alpha-olefins or 1-alkenes. In terms of lubricant property improvement, the thrust of the industrial research effort on synthetic lubricants has been toward fluids exhibiting useful viscosities over a wide range of temperature, i.e., improved viscosity index (VI), while also showing lubricity, thermal and oxidative stability and pour point equal to or better than mineral oil. These new synthetic lubricants lower friction and hence increase mechanical efficiency across the full spectrum of mechanical loads from worm gears to traction drives and do so over a wider range of operating conditions than mineral oil lubricants.
Catalysts that have been found useful in the prior art for the oligomerization of alpha-olefins to PAO include Lewis acids and Ziegler catalysts. The products have been found to differ significantly in lubricant properties according to the catalyst used and the process economics are also affected by ease of separation, corrosivity and other catalyst dependent process characteristics. Brennan, Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 2-6, cites 1-decene trimer as an example of a structure compatible with structures associated with superior low temperature fluidity wherein the concentration of atoms is very close to the center of a chain of carbon atoms. Also described therein is the apparent dependency of properties of the oligomer on the oligomerization process, i.e., cationic polymerization or Ziegler-type catalyst, known and practiced in the art.
A process using coordination catalysts to prepare high polymers from 1-alkenes, especially chromium catalyst on a silica support, is described by Weiss et al. in Jour. Catalysis 88, 424-430 (1984) and in Offen. DE 3,427,319. The process and products therefrom are discussed in more detail hereinafter in comparison with the process and products of the instant invention.
Recently, novel lubricant compositions (referred to herein as HVI-PAO and the HVI-PAO process) comprising polyalpha-olefins and methods for their preparation employing as catalyst reduced chromium on a silica support have been disclosed in U.S. patent application Ser. Nos. 210,434 and 210,435 filed June 23, 1988, now U.S. Pat. Nos. 4,827,073 and 4,827,064 incorporated herein by reference in their entirety. The process comprises contacting C.sub.6 -C.sub.20 1-alkene feedstock with reduced valence state chromium oxide catalyst on porous silica support under oligomerizing conditions in an oligomerization zone whereby high viscosity, high VI liquid hydrocarbon lubricant is produced having branch ratios less than 0.19 and pour point below -15.degree. C. The process is distinctive in that little isomerization of the olefinic bond occurs compared to known oligomerization methods to produce polyalpha-olefins using Lewis acid catalyst. Lubricants produced by the process cover the full range of lubricant viscosities and exhibit a remarkably high viscosity index (VI) and low pour point even at high viscosity. The as-synthesized HVI-PAO oligomer has a preponderance of terminal olefinic unsaturation.
Considering the abundance of C.sub.2 to C.sub.5 alpha-olefins in the petroleum refinery, and their low cost, it has long been been recognized that they could be a preferred source of low cost lubricant if they could be oligomerized to provide high viscosity index lubricant in good yield with a manageable, regenerable, non-corrosive catalyst. Accordingly, the objectives of the present invention include:
a novel process for the oligomerization of C.sub.3 to C.sub.5 alpha-olefins or mixtures of C.sub.2 to C.sub.5 alpha-olefins to produce a lubricant of superior quality;
a novel process for the oligomerization of C.sub.3 to C.sub.5 alpha-olefins or mixtures of C.sub.2 to C.sub.5 alpha-olefins that provides hydrocarbon lubricants and chemical intermediates;
and the foregoing novel process using a low cost catalyst that is essentially non-corrosive, readily recoverable and regenerable.